1. Field of the Invention
The present invention relates to a method and apparatus for improving the radiating pattern of a parabolic antenna and, more particularly, to eliminating the grating lobes of a confocal parabolic antenna system.
2. Prior Art
A known confocal parabolic antenna system is shown in FIGS. 1a, 1b and 1c. Such an antenna has been proposed by Dragone and Gans for a satellite communication system as described in "Imaging Reflector Arrangement To Form A Scanning Beam Using A Small Array", The Bell System Technical Journal, Vol. 58, No. 2, Feb. 1979. To utilize the reflector efficiently, it has been proposed to operate the antenna at both uplink (ground to satellite) and downlink (satellite to ground) frequency bands through the use of orthogonally polarized feed elements. Since the uplink frequency is considerably higher than the downlink frequency, the antenna will have grating lobes at the uplink frequency. It would be desirable to develop a technique which can reduce the grating lobes of such an antenna system. These are some of the problems this invention overcomes.
Dragone and Gans have continued their work described in "Satellite Phase Arrays: Use of Imagining Reflectors With Spacial Filtering In The Focal Plane To Reduce Grating Lobes", The Bell System Technical Journal, Vol. 59, No. 3, March, 1980. A filter, placed in the focal plane of the main reflector, eliminates undesirable field components due to the grating lobes of the small array. Because of the filter, the illumination over the main aperture is a smoothed version of the array illumination. Thus, grating lobes are reduced. By adjusting the excitation of the various antenna array elements, an antenna with low side lobes is obtained. However, the paper does not disclose the construction of a filter to accomplish any reduction of grating lobes.
Referring to FIGS. 5 through 9 of this article, it is shown that the teaching of the article is to reduce the grating lobes at both the uplink (14.25 GHz) and the downlink (11.8 GHz) frequencies. The reduction of the grating lobes at the downlink frequency is not controllable. It is the by-product of the reduction of the grating lobes at the uplink frequency with the expense of gain reduction at the downlink frequency. This is significant because if reduction of the gain is acceptable, the use of a filter is not necessary. A simpler, less expensive and lighter solution would then be to reduce the size of the subreflector illustrated in FIG. 1 of the publication. In practice, the reduction of the gain at the downlink frequency is more serious than the presence of the grating lobes at the downlink frequency since the grating lobes at the downlink frequency are located at greater angles from the main beam than the grating lobes at the uplink frequency. Thus, the important issue is to reduce the grating lobes at the uplink frequency without affecting the performance of the antenna at the downlink frequency. In the referenced article, there is no teaching as to how to reduce the grating lobes at one frequency without affecting the antenna performance at other frequency.
Further, the prior art teaches various ways of altering antenna radiation patterns. For example, a metallic grid will reflect the component of the electromagnetic wave parallel to the grid and will transmit the component of the electromagnetic wave perpendicular to the grid. The following patents use such a grid to eliminate the cross polarization content of a reflector antenna: U.S. Pat. Nos. 4,144,535 issued to Dragone; 4,119,932 issued to Bockrath and 4,109,253 issued to Chu.
A metallic grid placed in front of a metallic plate will not only reflect the incident electromagnetic wave, but also twist the polarization vector of the wave. Such a property is well known and is taught in the U.S. Pat. to Rogers, No. 3,797,020. U.S. Pat. No. 4,070,678 issued to Smedes utilizes such a device to provide a wide angle scanning antenna.
Metallic grids can also be used to control the direction of the main beam of the antenna thus controlling the direction of main energy flow. Such a configuration is taught in U.S. Pat. No. 3,797,020 to Roger; 3,771,160 to Laverick and 3,261,020 to Kay. Nevertheless, none of these patents teach eliminating or blocking the energy radiating into the direction of the grating lobes but are directed to changing the direction of the main energy beam.